linux-kernel - Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

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Message-ID: <4812CB99.1070600@goop.org>
Date:	Fri, 25 Apr 2008 23:28:41 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	npiggin@...e.de, torvalds@...ux-foundation.org,
	peterz@...radead.org, sam@...nborg.org, Jes Sorensen <jes@....com>
Subject: Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

Jens Axboe wrote:
> On Thu, Apr 24 2008, Russell King wrote:
>   
>> On Tue, Apr 22, 2008 at 08:50:17PM +0200, Jens Axboe wrote:
>>     
>>> +		data->csd.func(data->csd.info);
>>> +
>>> +		spin_lock(&data->lock);
>>> +		cpu_clear(cpu, data->cpumask);
>>> +		WARN_ON(data->refs == 0);
>>> +		data->refs--;
>>> +		refs = data->refs;
>>>       
>> Probably a silly question, but what does data->refs do that
>> cpus_empty(data->cpumask) wouldn't do?  (as indeed ARM presently does.)
>>     
>
> I guess it can be marginally slower for NR_CPUS > BITS_PER_LONG,
> otherwise there's absolutely no reason to have a seperate ref counter.
>   

Jes was concerned about scanning bitmasks on a 4096 CPU Altix.  I'm not 
sure its all that important, but a refcount check would definitely be 
quicker.

    J
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